Interpretive Summary: Estimates of the components that affect variation are required to calculate national genetic evaluations of dairy cattle. However, because of computational limitations, these parameters have never been estimated using the complete national data set of lactation records maintained by USDA. In this study, Method R, a relatively new procedure that allows analysis of large data sets, was used to estimate parameters using the same data and analysis model as are used for genetic prediction. Additionally, improvement in the prediction of genetic merit of dairy bulls as a result of using the new parameters was validated. Based on these results, an average heritability of .30 with a range of .25 to .35 is used for current USDA-DHIA genetic evaluations for yield traits. The improved parameter estimates and consequently more accurate evaluations will allow dairy breeders to increase genetic progress for desired traits.

Technical Abstract:
Heritabilities for milk, fat, and protein yields were estimated from first lactation data of Ayrshire, Brown Swiss, Guernsey, Holstein, Jersey, Milking Shorthorn, and Red and White dairy cattle used for USDA-Dairy Herd Improvement Association (DHIA) genetic evaluations. Contemporary group assignments and standard deviations within herd-year were determined with the procedure used for national evaluations. Pedigree data were included for animals born since 1970; yield data were included for cows born since 1980. Lactation records were divided into four mutually exclusive data sets based on standard deviation. Ranges for standard deviation were chosen so that data sets were approximately equal in size. Method R was used to estimate heritabilities with 25 different random samples of half of the data for each data set. Because of the large number of Holstein observations, estimates of heritabilities for Holsteins were based on random subsets of the complete data file; each subset included approximately 5% of the data. Mean heritability estimates increased with standard deviation, and estimates ranged from 0.18 to 0.51 across breeds. Repeatability estimates for milk yield of Holsteins were approximately 0.50 and did not change with standard deviation. These heritability estimates were higher than those previously used in the USDA-DHIA genetic evaluation, and heritabilities used in the genetic evaluation have been increased because of these results.